With reference to FIG. 1, for x-ray analysis in an electron microscope (EM) 100, an x-ray spectrum is measured by sensing and measuring the energies of individual x-ray photons emitted by a specimen 101 when it is hit by a focussed electron beam 102. Each x-ray photon is an energetic particle and the energy is typically converted into charge using a solid state detector 105. Electrons which are scattered back from the sample, so called “backscattered electrons” (BSE) 103, may also travel towards the x-ray detector. An electron with the same energy as an x-ray photon deposits the same amount of energy in the solid state detector and therefore gives a similar signal charge. Typically, the number of electrons travelling towards the detector is considerably larger than the number of x-ray photons so the signal due to electrons represents a large proportion of the measured x-ray spectrum and can overwhelm the contribution due to x-ray photons. The BSE contribution in the spectrum is typically a large background extending over all energies up to the primary beam energy.
The x-ray detector is usually isolated from the vacuum of the EM by a thin foil of typically polymer supported on a grid with high transparency. If BSE strike the foil or the grid they generate x-ray signals characteristic of the materials in the foil or grid and these x-ray signals appear as a spurious contribution to the recorded x-ray spectrum.
X-ray analysis may be conducted in principle in any apparatus using an incident beam of charged particles (e.g. electrons or ions) and although the following description refers to electrons, the same principles apply in any charged particle apparatus.
The objective of this invention is to find an improved means for preventing BSE from causing an undesirable contribution to the recorded x-ray spectrum, particularly for large area x-ray sensors. We firstly discuss some problems associated with existing methods.
As a result of the incident beam the X-ray photons are emitted in all directions and the x-ray detector only detects photons falling within a cone defined by the active area of the sensor within the x-ray detector. The higher the solid angle defined by this cone, the more signal is collected and this is highly desirable. The solid angle can be increased by increasing the active area of the detector so a large sensor and large aperture are beneficial.
The solid angle can also be increased by pushing the sensor closer to the specimen. However, when the sensor is placed closer to the specimen, the tube structure containing the sensor may collide with the final pole piece 104. This is shown by way of example at point A in FIG. 1. Therefore, it is desirable to minimise the external diameter of the detector tube (shown at B in FIG. 1) to allow the detector to be pushed closer to the specimen without colliding with the conical pole piece of the electron microscope.
Most existing solutions use a pair of permanent magnets to deflect the BSE so that they no longer travel towards the x-ray sensor. In U.S. Pat. No. 4,382,183 (Kimura), and with reference to FIG. 1 thereof, an opposing pair of permanent magnets are connected by a yoke 12. Electrons P, that would otherwise travel towards the sensor 6, are deflected into a region lying between the magnets and sensor where they fall on a grooved absorbing surface that prevents them scattering towards the sensor. A vacuum supporting window is shown at 7 in FIG. 1 of U.S. Pat. No. 4,382,183 and the grooves also help prevent any x-rays, generated by electrons hitting the absorber, from reaching the sensor. X-rays from the specimen 2 have an unrestricted path 1 to the sensor 6.
One problem with this arrangement is the length of the trap which prevents the sensor 6 being placed close to the specimen 2. An additional problem with magnetic electron traps is that the external field produced by the magnet influences the focussed electron beam in the EM. This problem is discussed in detail and is addressed by Ochiai et al in U.S. Pat. No. 6,653,637.
In U.S. Pat. No. 6,653,637, additional permanent magnets are introduced in order to cancel the external field that could influence the focussed electron beam. One embodiment describes an arrangement suitable for a circular detector sensor with area 10 mm2 where the maximum field for diverting electrons at the centre of the gap is 0.3 Tesla. A further embodiment provides a larger aperture sufficient to allow x-rays to fall on a sensor with area 30 mm2 where the maximum field is 0.2 Tesla for deflecting electrons and where the external field is small enough not to interfere with EM operation. However, if larger sensor is to be used, then the aperture must be increased still further. If the embodiment described by FIG. 6 of U.S. Pat. No. 6,653,637 is scaled to provide an aperture large enough to expose a sensor with active area of 80 mm2, without changing the length of the trap, then a series of undesirable effects occur:
a) the field between the magnets that deflects the electrons is substantially reduced;
b) the leakage field that influences the focussed electron beam is substantially increased; and,
c) the overall diameter of the external tube for the trap is increased and this makes it more difficult to put the sensor close to the specimen because of collision with the pole piece at points such as A in FIG. 1 herein.
It is problems such as those discussed above that the present invention addresses.